Optical module with a lens encapsulated within sealant and method for manufacturing the same

ABSTRACT

A method to manufacture an optical module is disclosed, wherein the optical module has an optically active device on a lead frame and a lens co-molded with the active device and the lead frame by a transparent resin as positioning the lens with respect to the lead frame. The molding die of the present invention has a positioning pin to support the lens during the molding. Because the lead frame is aligned with the molding die, the precise alignment between the active device on the lead frame and the lens is not spoiled during the molding.

TECHNICAL FIELD

The present invention relates to an optical module, in particular, theinvention relates to an optical module and a method for manufacturingthe module that encapsulates a lens with a transparent sealant.

BACKGROUND ART

The optical module to transmit and/or receive optical signals, generallyincludes a package that installs semiconductor optical devices, such asa semiconductor laser diode (hereafter denoted as LD) or a semiconductorphotodiode (hereafter denoted as PD), and an optical receptacle tocouple the semiconductor optical device with an external fiber.

One type of the optical module has an arrangement that the semiconductoroptical device is mounted on a lead frame, and both of them areencapsulated with a transparent resin. A Japanese Patent Applicationpublished as JP-2004-133117A has disclosed such an optical module thatencapsulates a lens, in addition to the optical device and the leadframe, within the sealant to compensate the degradation of the opticalcoupling between the optical device and the external fiber due to largertemperature dependence of the refractive index of the transparent resinto encapsulate the members. However, the lens encapsulated in thesealant is easily misaligned by the fluid resin during the molding,which increases the optical coupling loss between the device and theexternal fiber.

SUMMARY OF INVENTION

An aspect of the present invention relates to an optical module thatcomprises a semiconductor optical device, a lead frame, a lens, and asealant. The semiconductor optical device may be an LD or a PD. The leadframe mounts the semiconductor optical device. The lens is opticallycoupled with the semiconductor optical device. The sealant encloses thesemiconductor optical device, the lens, and a portion of the lead frame,where the sealant is transparent for light characterizing thesemiconductor optical device. A feature of the present invention is thatthe sealant has at least a via penetrating from a surface thereof to thelens.

Because the sealant has the via in a vicinity of the semiconductoroptical device in the present invention, heat generated by thesemiconductor optical device may be efficiently dissipated.

Another aspect of the present invention relates to a method tomanufacture an optical module. The method comprises steps of: (a)setting a lead frame on a molding die, where the lead frame mounts asemiconductor optical device thereon; (b) supporting a lens by a portionof the molding die in a position optically aligned with thesemiconductor optical device; and (c) molding the lens, thesemiconductor optical device, and the lead frame by a resin transparentfor light characterizing the semiconductor optical device.

Because the process according to the present invention supports the lensby, for instance, a positioning pin protruding within a cavity of themolding die, and the lead frame is also placed on the molding die; thelens may be aligned with the semiconductor optical device through themolding die. Moreover, because the lens is supported by the positioningpin, the alignment of the lens against the semiconductor optical devicemay be maintained during the injection of the resin into the cavity.

BRIEF DESCRIPTION OF DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a perspective view of an optical module according to anembodiment of the present invention;

FIGS. 2A to 2C are a plan view, a side view, and a front view of theoptical module shown in FIG. 1, wherein FIG. 2B illustrates an edge ofan external fiber;

FIG. 3 is a side cross section of dies used in the manufacturing of theoptical module shown in FIG. 1;

FIG. 4 is a perspective view showing a process to assemble the opticalmodule with a sleeve;

FIG. 5A is a perspective view of another optical module according to thesecond embodiment of the present invention, and FIG. 5B magnifies aprimary portion of the optical module shown in FIG. 5A;

FIG. 6 shows a process to form the optical module shown in FIG. 5A;

FIG. 7A is a side view showing a positioning pin and a lens according tothe second embodiment of the invention; and FIG. 7B shows the lens putbetween the positioning pins during the molding; and

FIG. 8A shows a modified embodiment of the positioning pin; and FIG. 8Bshows the process for supporting the lens between the positioning pins.

DESCRIPTION OF EMBODIMENTS First Embodiment

Next, preferred embodiments according to the present invention will bedescribed as referring to accompanying drawings. FIG. 1 shows an outerappearance of an optical module according to the first embodiment of thepresent invention; and FIGS. 2A to 2C show a plan view, a front view,and a side view of the optical module, respectively.

The optical module 1 of the present embodiment includes a light-emittingdevice, typically an LD 2 having a multi-quantum well active layer, alead frame 3, a lens 4, a sealant 5, and a sub-mount 6. The sealant 6includes a lens portion 10 and vias 7, details of which will bedescribed later. The lens portion 10 is formed in an outer surface ofthe sealant 5 in a portion crossing an axis connecting the LD 2 with thelens 4

The lead frame 3, typically made of copper or copper alloy, has asubstantially rectangular shape. The lead frame 3 mounts the LD 2through the sub-mount 6 thereon. One side of the lead frame 3 includes aplurality of lead terminals 3 a, which will be finally separated to eachothers; while, the other side of the lead frame includes a cut 3 b witha semi-circular shaper. The LD 2 is wire-bonded to one of lead terminals3 a. The cut 3 b receives the lens 4 so as to align the lens 4 opticallyon the axis of the LD 2.

The lens 4, which may be made of material with refractive index greaterthan that of a material for the sealant 5, has convex surface toconcentrate light emitted from the LD 2. Specifically, the lens 4 may bea spherical lens made of BK7 or TAF3 with a diameter of 0.5 to 1.5 mm.Further, the lens 4 may position such that a distance L_(1a) from thelens 4 to the LD 2 is set to be 0.3 mm, while, another distance L_(1b)from the lens 2 to the lens portion 10 is set to be 2.4 mm. Exactly, theformer distance L_(1a) is from the surface of the lens 4 closest to theLD 2 to the front edge of the LD 2, while, the latter L_(1b) is from thesurface of the lens 4 closest to the lens portion 10 to the top of thelens portion 10. These distances are an exemplary of the optical design;and may vary depending on the characteristics of the LD 2, the lens 4,and the lens portion 10.

The shape of the sealant 5 is a pillar having an axis substantially inparallel to the axis connecting the LD 2 with the lens 4. The pillar mayhave a diameter of 5 to 20 mm and a length 5 to 20 mm. The sealant 5,for instance, may be made of resin, such as epoxy resin, substantiallytransparent in wavelengths of the LD 2. The sealant 5 seals at least thelens 4 and a portion of the lead frame 3 where the LD 2 is mounted. Thesealant 5 has two vias 7 each penetrating from the top and bottomsurfaces, respectively, to the lens 4. The shape of the via 7 is atruncated cone.

The sealant 5 has the lens portion 10 in one end thereof. The lensportion 10 may concentrate light emitted from the LD 2 and transmittedthrough the lens 4. The sealant has the refractive index of 1.51 lessthan that of the lens 4. The optical axis of the lens portion 10 isaligned with the axis of the lens 4, that is, the sealant 5 is formedsuch that the axis of the lens portion 10 aligns with the axis of thelens 4. A distance L₂ from the lens portion 10, exactly, the top of thelens portion 4, to the ferrule 18 is set to be 2.4 mm in the presentembodiment.

Next, a manufacturing method of the optical module 1 will be describedas referring to FIG. 3. First, details of the molding die 11 for thetransfer molding will be described. The die 11, as shown in FIG. 3 thatshows a cross section of the die, includes an upper die 80 and a lowerdie 90. The lower die 90 includes a concaved surface 90 e, a supportterrace 90 f where the lead frame 3 is set thereon, and a positioningpin 90 h. The concaved surface 90 e forms a portion of the lens portion10. The positioning pin 90 h, which may set the position of the lens 4by supporting it from the lower side, has a shape of a truncated coneprotruding from the bottom of the lower die 90. The top of thepositioning pin 90 h has a concave surface that fits with an outer shapeof the lens 4. The lens 4 is placed on the positioning pin 90 h.

The upper die 80 has a concaved surface 80 e and a positioning pin 80 h.The concaved surface 80 e of the upper die 80, continuous from theconcaved surface 90 e of the lower die 90, forms the lens portion 10.The positioning pin 80 h, which has a mirrored shape with respect to thepositioning pin 90 h of the lower die 90, may align the lens 4 in apreset position.

The positioning pins, 80 h ad 90 h, put the lens 4 therebetween;specifically, the positioning pin 90 h of the lower die 90 supports thelens 4 from the bottom, while, the other positioning pin 80 h in theupper die securely positions the lens 4 from the upper by pressing thelens against the lower positioning pin 90 h. The tip of the positioningpins, 80 h and 90 h, are precisely formed so as not to cause anymechanical damage against the lens 4. The positioning pins, 80 h and 90h, may have a mechanism to move in up-and-down and be processed in waterrepellant in surfaces thereof.

Next, a process to form the sealant 5 by the transfer molding will bedescribed. First, the lead frame 3 is set on the support terrace 90 f ofthe lower die 90. Then the lens 4 is placed on the top of the lowerpositioning pin 90 h, in this instance, the lens 4 is supported by thelower positioning pin 90 h from the bottom, while, it is positioned bythe cut 3 b of the lead frame 3 from the side. Then, fitting the upperdie 80 with the lower die 90, the lens 4 may be supported and positionedby the upper positioning pin 80 h.

Mounting the die 11 on an apparatus of the transfer molding, andperforming the transfer molding; the cavities, 80 g and 90 g, may befilled with resin. Because the sealant 5 formed by the transfer moldingoften includes voids therein, the transfer molding of the presentinvention may be carried out such that the cavities, 80 g and 90 g, arefirst depressurized; subsequently, fluid resin is pressure-injected.

Preferable conditions of the transfer molding are follows, but unlimitedto those conditions:

-   -   Tightened force for the molding dies: 1.0 to 1.6×10³ kg,        preferably 1.0 to 1.2×10³ kg;    -   Curing time: 3 to 10 minutes, preferably, 5 to 7 minutes; and    -   Curing temperature: 155 to 180° C., preferably, 155 to 165° C.        After the molding, the optical module 1 may be removed from the        dies, 80 and 90. Because the positioning pins, 80 h and 90 h,        are processed in water repellant in surfaces thereof; the module        1 may be easily removed from the dies, 80 and 90. The module 1        is preferably set under a condition of a temperature of 150 to        165° C. for several hours to harden the resin further.

The optical module 1 thus transfer-molded may align the lens 4 opticallywith respect to the LD 2 mounted on the lead frame 3, because the lens 4is supported by three members of two positioning pins, 80 h and 90 h,and the cut 3 b of the lead frame 3 during the molding. Tri-pointssupporting described above may prevent the lens from being affected by aflow of the fluid resin. Thus, the lens 4 may be precisely aligned withthe LD 2 even the lens 4 has a body different from the resin.

The present embodiment supports the lens 4 by three members of twopositioning pins, 80 h and 90 h, and the cut 3 b of the lead frame 3.However, only two positioning pins, 80 h and 90 h, may support the lens4 during the molding, when the cut 3 b is hard to be provided by thearrangement of the LD 2 on the lead frame 3 and the melted resin hasviscosity not to affect the position of the lens 4.

The optical module 1 thus processed, as illustrated in FIG. 1, two vias7 which are cast-offs of the positions pins, 80 h and 90 h, are formedin the sealant 5 in positions symmetry with respect to the lens 4. Theoptical module 1 of the present embodiment is preferable to fill thevias 7 with a member having a good thermal conductivity to acceleratethe head dissipation; because the LD 2 generates heat and the sealant 5made of transparent resin has relatively lesser thermal conductivity.The heat generated by the LD 2 may accumulate around the LD 2. Moreover,the LD 2 is a device whose performance strongly depends on thetemperature. Accordingly, the member filling the vias 7 may acceleratethe heat dissipation from the LD 2 to the outside of the sealant 5. Themember may have a shape tracing the inner shape of the vias 7 and fixedthereat by an adhesive. The member may be made of metal such as aluminumor copper.

In such an arrangement of the heat dissipating member in the vias 7, theheat generated by the LD 2 may be not only conducted to the sealant 5around the LD 2 but transferred to the heat dissipating member, whichmay suppress the degradation of the LD 2 in a long term by beingaffected from the heat generated by itself.

Next, an optical coupling mechanism of the optical module 1 of thepresent embodiment will be described as referring to FIG. 4. The opticalmodule 1, as shown in FIG. 4, is set within a sleeve 17, and the sleeve17 may receive ferrule 18 therein to couple an external optical fibersecured in a center of the ferrule 18 optically with the LD 2.

The sleeve 17 may be made of metal, such as stainless steel, or resinsuch as polyetherimide. One of bores of the sleeve 17 that receives theoptical module 1 therein has a shape fit with the pillared shape of theoptical module 1, and an adhesive such as epoxy resin may fix theoptical module 1 within the bore of the sleeve 17. Thus, the opticalmodule 1 may be optically aligned with the external fiber within a gapbetween the outer surface of the sealant 5 and the bore of the sleeve17.

Next, the optical coupling loss was compared for optical modules withvarious arrangements. The first module fixed the lens 4 to the leadframe 3 by an adhesive then it is molded, the second one supported thelens 4 only by the cut 3 b of the lead frame 3 without any positioningpins during the molding, and the third one supported the lens 4 by thecut 3 b and two positioning pins, 80 h and 90 h, during the molding.Three types of the module mentioned above were compared by the opticaloutput power when a preset current is supplied to the LD 2. The firstmodule indicated an average optical output of about 0.4, the second oneindicated 0.7 and the third module was about 0.9, where they assumed theoptical output obtained from the bared LD supplied with the presetcurrent is 1.0.

Thus, the optical module 1 according to the present embodiment positionsthe lens 4 by three members, namely two positioning pins, 80 h and 90 h,and the cut 3 b of the lead frame 3; accordingly, the lens 4 may beprecisely aligned with the LD 2 on the lead frame 3. The mechanism toposition the lens 4 with respect to the LD 2 is not restricted to thosearrangement described above. Only two members, namely, two positioningpins, 80 h and 90 h, without the cut 3 b may position the lens 2 withsufficient accuracy. Moreover, only the lower positioning pin 90 h mayalign the lens 4.

Second Embodiment

Next, another optical module according to the second embodiment of thepresent invention will be described as referring to FIGS. 5A and 5B,where FIG. 5A magnifies a primary portion of the optical module 1A,while, FIG. 5B removes the sealant 5 shown in FIG. 5A. The opticalmodule 1A shown in FIGS. 5A and 5B also includes, as those of theaforementioned module 1, an LD 2, a lead frame 53, a lens 4 and asealant 5. The lead frame 53 mounts the LD 2 thereon and has a cut 53 bin one end thereof within which the lens 4 is installed. The lead frame53 also has two holes 53 e in both sides of the cut 53 b putting theoptical axis of the LD 2 and the lens 4 therebetween. The objects of theholes 53 e will be described later in this specification.

In the present embodiment, the vias for arranging the lens 4 are formedsuch that the upper side of the sealant 5 with respect to the lead frame3 has two vias 5 a with a rectangular cross section in both sides of thelens so as to continue to the holes 53 e; while, the lower side of thesealant 5 also has two vias 5 b but with a circular cross section so asto continue to the holes 53 e. These vias, 5 a and 5 b, may be formedduring the molding to form the sealant 5. The sealant 5 of the presentembodiment also installs a lens portion 10 in an outer surface crossingthe optical axis of the LD 2 and the lens 4. The lens portion 10 is atype of the aspherical lens whose optical axis is aligned with the axisof the lens 4.

A method to form the sealant 5, that is a method to encapsulate the LD 2and the lens 4, will be described.

FIG. 6 is a process to mold the sealant 5 according to the presentembodiment. FIG. 6 views the lens 4 and the lead frame 53 from the lowerside thereof, that is, FIG. 6 shows a back surface of the lead frame 53,where the lead frame 53 is set within the molding die including theupper die 89 and the lower die, the latter of which is not illustratedin FIG. 6.

The upper die 89 shown in FIG. 6 has two positioning pins 89 a eachhaving a hollow 89 b in an end portion thereof. The hollow 89 b inrespective positioning pins 89 a faces to each other. The bottom portionof the hollow 89 b is spherical so as to fit the outer shape of the lens4; accordingly, the positioning pins 89 a may securely support the lens4 during the transfer molding. A top of the positioning pin 89 a has aprojection 89 c to be inserted within the hole 53 e of the lead frame 53to align the positioning pins 89 a with the LD 2 on the lead frame 53.

The lower die also provides two positing pins having a truncated coneshape such as those shown in FIG. 3. The top of the positioning pin ofthe lower die forms a hole that receives the projection 89 c of theupper positioning pin 89 a. Thus, the upper positioning pin 89 a may besecurely aligned with the lead frame 53 during the molding. The surfaceof the upper and lower positioning pins are processed in water repellantto enhance the detachment of the molding from the sealant 5 after themolding.

The method to form the sealant 5 according to the present embodimentwill be described. The process first sets the lens 4 within the hollow89 b of the upper positioning pin 89 a, then sets the lead frame 53, onwhich the LD 2 is mounted through the sub-mount 6, on the upperpositioning pin 89 a so as to insert the projection 89 c into the hole53 e. The lens 4 may be optically aligned with the LD 2 on the leadframe 53. Then, the lower die is joined with the upper die 89 such thatthe hole in the top of the lower positioning pin engages with theprojection 89 c of the upper positioning pin 89 a; thus, the lead frame53 may be securely supported between two dies. Finally, the processinjects a transparent resin within the cavity formed between two dies.Conditions of the molding are substantially same as those of the formerembodiment.

FIG. 7A magnifies a tip of the upper positioning pin 89 a viewed fromthe side thereof, while, FIG. 7B shows two upper positioning pins 89 aputting the lens 4 therebetween to support it during the molding, whichis viewed from the top of the positioning pin 89 a. As shown in FIG. 7A,the bottom portion 89 d of the hollow 89 b has a spherical shape to fitthe outer shape of the lens 4. Moreover, the side 89 e of the hollow 89b also has a spherical shape to fit the outer shape of the lens 4. Thus,the tip of the positioning pin 89 a may securely support the lens 4during the molding by the shape fitting the outer shape of the lens 4.The positioning pin 89 a according to the present embodiment includestwo spherical surfaces, 89 d and 89 e, within the hollow 89 b; however,the positioning pin 89 a may have only one of the spherical surfaces, 89d and 89 e, to support the lens 4.

FIGS. 8A and 8B magnify the tip portion of the upper positioning pinaccording to a modified embodiment of the invention. The upperpositioning pin 99 a of the modified embodiment has a hollow 99 b in thetip thereof but the hollow 99 b includes a plurality of plane surfacesin a bottom portion thereof, that is, the vertical cross section thereofshown in FIG. 8A is polygonal. At least three surfaces of the hollow 99b may come in contact with the lens 4 so as to support it securely.Moreover, as shown in FIG. 8B, the hollow 99 b may have a horizontalpolygonal shape with a plurality of plane surfaces, at least three ofwhich come in contact with the lens 4. Thus, the arrangement of thehollow 99 b according to the modified embodiment may securely supportthe lens 4 during the molding. The polygonal shape is not restricted tothose illustrated in FIGS. 8A and 8B. For instance, a V-shaped groovemay also support the lens, where two surfaces come in contact with thelens 4. Moreover, at least one of the bottom portion and the sideportion of the hollow 99 b may have a polygonal shape to support thelens 4.

The molding process thus described may securely support the lens 4 whichis encapsulated by the resin during the molding by two positioning pins,which are parts of the molding dies; accordingly, the lens may be hardlyaffected in the position thereof by the fluid resin and may maintain theoptical alignment with respect to the LD 2 on the lead frame 53.

Also in the present embodiment, metal members, such as copper oraluminum, may fill the vias, 5 a and 5 b, in the sealant 4 after themolding to enhance the heat dissipation from the LD 2 through thesealant 5 and the metal members in the vias, 5 a and 5 b. The presentembodiment provides four vias close to the LD 2, which further enhancesthe heat dissipation comparing with those of the former embodiment thathas two vias.

The optical module 1 according to the present embodiment may considerthe temperature dependence of the refractive index of the lens 4 andthat of the sealant 5, because the refractive index slightly shifts,depending on the temperature, the focal point and resultant opticalcoupling efficiency between the LD 2 and the external fiber.Accordingly, the optical module 1 designs the distance L_(1a) from theLD 2 to the lens 4, another distance L_(1b) from the lens 4 to the lensportion 10 of the sealant 5, and the radius R of the lens portion 10according to the refractive index and the diameter of the lens 4.

Various dimensional conditions of the lens 4, the lens portion 10 andthe LD 2 were verified under a wide range of the temperature by theoptical coupling loss ΔPf and the tracking error (T.E) thereof asindices. The T.E. means a ratio of the maximum to the minimum of theoptical coupling efficiency within a temperature range from −40 to 95°C., and preferably to be less than 0.5 dB. Table 1 shows the result ofthe T.E. under conditions where the emission wavelength of the LD 2 is1310 nm, the refractive index of the resin for the sealant 5 is 1.51 at25° C. with a thermal co-efficient of −1.3×10⁻⁴/° C., and the corediameter of the external fiber exposed in the tip end of the ferrule is10 μm.

The evaluation described below first assumes the diameter of the lens 4from 0.4 to 1.8 mm; then other dimensional parameters of distances,L_(1a), L_(1b) and L₂, and the radius R of the lens portion 10 areselected such that the optical coupling between the LD 2 and theexternal fiber at the tip end becomes a maximum.

TABLE I D(mm) 0.4 0.6 0.8 1.0 1.2 1.8 L_(1a)(mm) 0.174 0.227 0.311 0.3790.428 0.603 L_(1b)(mm) 0.465 0.350 0.541 0.583 0.446 0.407 R(mm) 0.300.36 0.50 0.60 0.65 0.90 L₂(mm) 1.404 1.760 2.425 2.934 3.240 4.553T.E.(dB) 1.51 0.049 0.43 0.34 0.31 0.16 ΔPf average good good good goodbad

In the whole evaluations, a material of the lens preferably has therefractive index thereof greater than that of the sealant 5. In thepresent embodiment, the lens 4 may be made of TaF3 whose refractiveindex is 1.78. The results are:

(1) a lens with a diameter D less than 0.6 mm is not only hard to beplaced on the top of the positioning pin or put between the pins buteasily misaligned during the molding due to the lightness thereof, eventhe optical modules shows a moderate or average loss for the opticalcoupling ΔPf; and

(2) oppositely, a lens with a diameter of 1.8 mm or greater accompanieswith greater dimensional parameters, L_(1a), L_(1b) and L₂, whichrequests the longer sleeve even if the precise optical alignment of thelens 4 becomes available.

The longer sleeve inherently shows less tolerance during the assemblythereof; in particular, when such a sleeve is fixed with an adhesive,the optical alignment is easily deformed. The table 1 above shows arelatively better tracking error T.E. for case of the lens diameter of1.8 mm; however, the optical coupling ΔPf was unacceptable. Accordingly,a lens 4 for the optical module 1 of the invention preferably has adiameter from 0.6 to 1.2 mm.

While several embodiments and variations of the present invention aredescribed in detail herein, it should be apparent that the disclosureand teachings of the present invention will suggest many alternativedesigns to those skilled in the art.

We claim:
 1. An optical module, comprising: a semiconductor opticaldevice; a lead frame for mounting said semiconductor optical device,said lead frame having a cut; a lens optically coupled with saidsemiconductor optical device, said lens being set within said cut ofsaid lead frame; and a sealant for enclosing said semiconductor opticaldevice, said lens and a portion of said lead frame, said sealant beingtransparent for light characterizing said semiconductor optical device,wherein said sealant has at least a via penetrating from a surface ofsaid sealant to said lens.
 2. The optical module of claim 1, whereinsaid via has a cross section of a truncated cone.
 3. The optical moduleof claim 1, wherein said via has a cross section of a trapezoid.
 4. Theoptical module of claim 1, wherein said via is filled with a metal. 5.The optical module of claim 1, wherein said sealant has an opticallyactive outer surface in a portion crossing an axis connecting saidsemiconductor optical device to said lens.
 6. The optical module ofclaim 1, wherein said sealant has a pair of vias putting an axisconnecting said semiconductor optical device with said lenstherebetween.
 7. The optical module of claim 1, wherein said via passesthrough said sealant to expose said lens in a midway of said via.